Apoptosis is a process used by higher organisms to maintain homeostasis by removing cells that are in excess, damaged, or potentially dangerous. Caspase enzymes are a class of cysteine proteases that cleave cellular substrates after recognition sequences with C-terminal aspartate residues. The activation of caspase enzymes is critical for apoptosis. There are two canonical apoptotic pathways, differing in that the apoptosis-initiating stimulus is intracellular (intrinsic pathway) or extracellular (extrinsic pathway). These pathways converge at the cleavage of procaspase-3 to form the active caspase-3, the key “executioner” caspase that catalyzes the hydrolysis of hundreds of protein substrates, leading to cell death.
One of the hallmarks of cancer is the ability of cancer cells to evade apoptosis, allowing for unchecked proliferation. As such, reactivation of apoptosis in cells with defective apoptotic pathways is a promising anticancer strategy. Compounds such as p53-MDM2 disruptors (Nutlins), Bcl-2 inhibitors (ABT-737), and inhibitors of XIAP (SM-164) all act directly on proteins in the apoptotic cascade, inducing apoptosis and leading to death of cancer cells.
Complementary to the strategies described above, the direct activation of procaspase-3 with a small molecule has potential for personalizing cancer therapy. Procaspase-3 levels are elevated in certain cancers, including lymphomas, leukemias, melanomas, pancreatic cancer, liver cancers, lung cancers, breast cancers, and colon cancers. Due to the elevated levels of procaspase-3 in cancer cells, the requirement of caspase-3 activation for apoptosis, and the relative downstream location of procaspase-3 in the apoptotic cascade, induction of apoptosis by the direct activation of procaspase-3 is being actively explored as a personalized anticancer strategy. Accordingly, there is a need for new compounds that modulate procaspase-3 activity, particularly compounds that activate procaspase-3 and that are metabolically stable enough to be effective clinical therapies.